setup VPN on OS 13

Settings->General->VPN and enter the required settings for the VPN.

That would depend greatly upon what you’re trying to achieve - and whether you are hosting the VPN server, or relying upon a third-party VPN host for your connection. Likewise, what VPN connection protocol are you intending to use - a natively supported VPN protocol (such as IKEv2 or IPsec) or a proprietary protocol (e.g., OpenVPN) for which you will require a third-party client App and server.

So, what is your security target?

Any VPN you can purchase, or is free are anything but private. Everything you do goes through their server and you have no way of knowing what they're doing with that information.

The only secure VPN is one you setup yourself on your own server.